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Viruses consist of a nucleic acid surrounded by one or more proteins. Some viruses also have an outer-membrane envelope. Viruses are obligate intracellular parasites: they can replicate only within cells since their nucleic acids do not encode the many enzymes necessary for protein, carbohydrate, or lipid metabolism and for the generation of high-energy phosphates. Typically, viral nucleic acids encode proteins necessary for replicating and pack–aging the nucleic acids within the biochemical milieu of host cells.

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Viruses differ from virusoids, viroids, and prions. Virusoids are nucleic acids that depend on helper viruses to package their nucleic acids into virus-like particles. Viroids are naked, cyclical, mostly double-strand, small RNAs that appear to be restricted to plants, spread from cell to cell, and are replicated by cellular RNA polymerase II. Prions (Chap. 383) are abnormal protein molecules that can spread, reproducing by changing the structure of their normal cellular protein counterparts. Prions have been implicated in neurodegenerative conditions such as Creutzfeldt-Jakob disease, Gerstmann-Straüssler disease, kuru, and human bovine spongiform encephalopathy ("mad cow disease").

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Viral genomes consist of (1) a single-strand or double-strand DNA, (2) a single-strand sense RNA, (3) a single-strand or segmented antisense RNA, or (4) a double-strand segmented RNA genome. The viral nucleic acid may encode only a few genes or more than 100. Sense-strand RNA genomes can be translated directly into protein, whereas antisense RNAs must be copied into translatable RNA. Sense and antisense genomes are also referred to as positive-strand and negative-strand genomes, respectively. Viral nucleic acid is usually associated with one or more virus-encoded nucleoproteins in the core of the viral particle. The viral nucleic acid and nucleoproteins are almost always enclosed in a protein shell called a capsid. Because of the limited genetic complexity of viruses, their capsids are usually composed of multimers of identical capsomeres. Capsomeres are in turn composed of one or a few proteins. Capsids have icosahedral or helical symmetry. Icosahedral structures approximate spheres but have two-, three-, and fivefold axes of symmetry, while helical structures have only a twofold axis of symmetry. The entire structural unit of nucleic acid, nucleoprotein(s), and capsid is called a nucleocapsid.

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Many human viruses are composed simply of a core and a capsid. For these viruses, the outer surface of the capsid mediates contact with uninfected cells. Other viruses are more complex and have an outer lipid-containing envelope derived from virus-modified membranes of the infected cell. The piece of infected-cell membrane that becomes the viral envelope has usually been modified during infection by the insertion of virus-encoded glycoproteins, which mediate contact of enveloped viruses with uninfected cells. Matrix or tegument proteins fill the space between the nucleocapsid and the envelope in many enveloped viruses. In general, enveloped viruses are sensitive to lipid solvents and nonionic detergents that can dissolve the envelope, while viruses that consist only of nucleocapsids are somewhat resistant. A schematic diagram for large and complex herpesviruses is shown ...

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